"S-13. Nepheline. Bancroft, Ontario (19). This sodium-potassium
aluminum silicate, (Na,K)(Al,Si)2O4, is found in igneous rocks
whose magmas contained insufficient silica to combine with the soda
present to form feldspar. This nepheline sample illustrates very
clearly the effect of grinder steel contamination. Note that the two
contaminated finer particle size fractions have both reduced
reflectivity and their near-infrared bands eliminated. In contrast, the
uppermost curve of uncontaminated 0.5µ nepheline is both highly
reflective and retains its spectral features. The bands near 1.4, 1.9
and 2.5µ are due to water, presumably present as fluid inclusions in
microscopic vesicles within the nepheline. The water is not present as
part of the nepheline lattice, and the only contaminant (a very small
amount of biotite) would not produce these bands."

Comment: HS-19.3 gives an exceptionally well-crystallized pattern. Unindexed
reflections at 5.64, 4.03, 3.44, 2.94, and 2.81 may prove to be nepheline peaks
that are too weak to be reported in the literature.